The present invention relates to a method designed to produce and treat wood fibers.
One problem which is generally encountered in connection with plants for producing and treating wood fibers is the emission to the surrounding atmosphere of volatile organic substances, i.e. Volatile Organic Compounds (VOC), as well as formaldehyde from the raw wood, and from the size which is used in the process. One object of the present invention is to solve this problem. Another object of the present invention is to recover thermal energy in these processes.
These and other objects have now been accomplished by the invention of apparatus for the treatment of wood chips comprising a preheater for preheating the wood chips in the presence of steam thereby producing volatile organic substances from the wood chips therein, a beater for mechanically processing the wood chips whereby wood fibers are released from the wood chips and the steam and the volatile organic substances are contained therein, a cyclone for separating the wood fibers from the steam and the volatile organic substances, the cyclone including an inlet, an upper outlet and a lower outlet, a blower line for transporting the wood fibers, the steam and the volatile organic substances from the beater to the inlet of the cyclone, a drying conduit connected to the lower outlet of the cyclone for carrying the wood fibers therefrom, a sluice valve associated with the lower outlet of the cyclone for controlling the removal of the wood fibers from the cyclone, and a processing conduit connected to the upper outlet of the cyclone for the steam and the volatile organic substances whereby the volatile organic substances separated in the cyclone can be separated and heat associated with the steam separated in the cyclone can be recovered. In a preferred embodiment, the apparatus includes a scrubber for separating the volatile organic substances from the steam separated in the cyclone, and a heating coil for recovering heat from the steam separated in the scrubber.
In accordance with a preferred embodiment of the apparatus of the present invention, the apparatus includes a steam ejector associated with the lower outlet of the cyclone for transporting the wood fibers in the dryer conduit, and a mixing chamber attached to the steam ejector for mixing flue gas and drying air and supplying the mixture of flue gas and drying air to the steam ejector.
In accordance with one embodiment of the apparatus of the present invention, the apparatus includes an injector associated with the lower outlet of the cyclone, and an air compressor for supplying compressed air to the injector for transporting the wood fiber, the steam and the volatile organic substances in the drying conduit.
In accordance with another embodiment of the apparatus of the present invention, the preheater includes an upper portion including a wood chip entrance and an outlet, a lower portion, and a steam inlet for supplying steam to the lower portion of the preheater whereby the steam can countercurrently contact the wood chips entering the wood chip entrance in the preheater and the volatile organic substances and steam can exit from the outlet from the preheater, and a scrubber connected to the outlet from the preheater whereby the volatile organic substances can be separated from the steam and additional heat can be recovered from the steam.
In accordance with another embodiment of the apparatus of the present invention, the apparatus includes a heat exchanger connected to the outlet from the cyclone and a condensate tank connected to the heat exchanger whereby the volatile organic substances can be separated in the gaseous state for incineration and the heat can be recovered from the steam. Preferably, the heat exchanger comprises a first heat exchanger, and the apparatus includes a second heat exchanger and a third heat exchanger connected to the condensate tank for providing heat for drying the wood fibers and for cooling the condensate in the condensate tank. Preferably, the apparatus includes at least one mixing chamber whereby the drying conduit supplies drying air to at least one of the second and third heat exchangers.